IRCC Award Talk Si Quantum Dots for Solar Cell … of Energy Science and Engineering Indian...

Department of Energy Science and Engineering

Indian Institute of Technology Bombay

Si Quantum Dots for Solar Cell Applications

Chetan S. Solanki

IRCC Award Talk

18th Aug. 2010

Acknowledgements Dr. Ashish Panchal

Dharmendra Rai

Paresh Kale

Dr. Pravin Narwankar, Applied Materials

Department of Energy Science and Engineering

Applied Materials

Nano-electronics Project

My family

Chetan S Solanki, IIT Bombay 3

Contents Solar PV scenario: World and India

Why Si nanomaterials?- Solar PV potential, Cost and efficiency

Si quantum dots for solar cells

- All-Si multi-junction cells

- Obtaining Si QDs: HWCVD and Porous Si

- Results and Analysis

Conclusion and future work

Generation of Power with Solar Cell• A Solar cell is a device that convert light into electricity

It requires

- Absorption of a photon

- Separation of a electron-hole pair

- Collection of the charges at electrodes

P-N Jn –separation force

Metal contact

Metal contact

• Sun light as input energy

• No moving parts, long life, moderate efficiency

• Solar cell are being manufactured since 1954

Solar Energy Potential in India 300 days of clear sky average 2000 kWh/m2/year

Our capita energy consumption is about 600 kWh/capita/year

India’s annual energy consumption is about 4 trillion kWh/year

India’s solar energy resource is 5000 trillion kWh/year

Characteristics of Solar PV technologies are:

- Clean, maintenance free

- Modular

- Distributed generation

Worldwide PV Module Production

PV market is growing with over 35% rate since last decade

Learning curve for PV is -18%

Solar PV in India


Current cell and module manufacturing capacity is about 400 and 750 MWp respectively

Jawaharlal Nehru National Solar Mission (JNNSM) has been announced to promote solar PV electricity generation in India

SIPS scheme for promoting solar PV manufacturing in India

Target is to install 20,000 MW of Solar power in India by 2022 stirred lot of activities in the country

The Mission document mentions NCPRE at IIT Bombay: “setting up of a National Centre for Photovoltaic Research & Education

at IIT Bombay, drawing upon its Department of Energy Science & Engineering and its Centre of Excellence in Nanoelectronics”

Solar PV Module Prices

Source: www.solarbuzz.com

Price fluctuation due to demand-supply balance, increase in production volume

Cost of conventional power is ~1$/Watt

Solar PV technology is expensive

Si Shortage

Challenges to PV Technologies

High price per unit watt (high cost of material)

1.5 to 4 $ / Watt, should be about 1$/Watt

Moderate efficiencies 14 to 16% for c-Si, 6 to 9% for thin film, higher is better

Availability of material Should be abundant

Long term stability Minimum acceptable life is 25 years

Long energy pay back period (high processing cost)

2 to 3 years for c-Si, < 1 year for thin-film, should be low

Long money pay back period Depends on region, 5 to 12 years

Technology Attributes Features


Efficiency and Cost

in

scoc

P

FFIVEfficiency =η

• Efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun.

IscI

Vm

Im

Pm

X

Voc

Power

===2

2$

$

mWatt

mWatt

Cost Production cost

Efficiency

Raw material cost, cell and module processing

Quality of material, technology understanding, cell size


Photovoltaic Generations: 1st, 2nd & 3rd

1st generation: Si wafer based technologies

3rd generation: Advanced nanostructure based concepts

2nd generation: Thin-film technologies

0 100 200 300 400 500

Cost, US$/m2

80

60

40

20

Effi

cien

c y, %

100

US$0.10/W US$0.20/W US$0.50/W

US$1.0/W

US$3.50/W

III

III

Ref: M.A Green, Progress In Photovoltaics, 9 (2000) 123.


Si is good but expensive

How to minimize Si consumption?


Si is Expensive

Metallurgical grade Si (MGS)

Initial

ReactionCholorosilanes

Separation and

purificationPure SiHCl3

Deposit solid Si

H2

Pu

re p

oly

-E

GS

EGS ingot

Grow single crystal

Si wafers

HC

l

Solid

Gas Gas

Solid

LiquidSolid

QuarziteCoal +Liquid

Melting

Solid

Pu

re p

oly

-E

GS

http://en.wikipedia.org/wiki/Image:Sand_patterns.jpg


Approaches to Si PV cost reduction

High cost due to large volume consumption of high purity material

Si

Concentrators

Thin film C-Si

Nanostructures of Si


Contents Why Si nanomaterials

- Solar PV potential- Cost and efficiency

Si nanomaterials for solar cells

- All-Si multi-junction cells

- Obtaining Si QDs: HWCVD and Porous Si

- Results and Analysis

Conclusion and future work


Concept of multi-junction cells

How to obtain higher efficiency?


Single junction cells are inefficient

200 J 50 J100 J 100 J

Required work


18

Single junction cells are inefficient

Losses in energy conversion inefficient utilization of solar spectrum

1-transmission, 4- contact losses

2-thermalization, 5. recombination losses

3-junctionn loss

23%

33%

energyPhoton

doneWork

EffPhoton

=

.

Eg

Electrons

Holes

Photon efficiency is different for different energy


Strategies for higher efficiencyApproach- I Band gap matching with spectrum (splitting spectrum over several

materials/cells)• tandem cells • intermediate band cells

Approach- III: Reshaping the solar spectrum

(Adopting Solar spectrum for one host material)

up- and down-conversion

Approach- II: Reduced thermalization losses (adopting one host material for solar spectrum)

hot electron cells

0.5 1 1.5 2 2.5Wavelength (µm)

0.5

1.0

1.5

Su

nlig

ht

inte

ns i

ty (

kW/m

2/µ

m)

Multi-junction cell approach


All Si Multi-junction solar cells

How to change band gap?


Quantum confinement and Band gap

D Si D Si D Si

Quantum confinement occurs when the crystal size becomes less than the Bhor exciton radius (4.9 nm for Si)

SiO2, SiN, SiC could be used as possible dielectric matrix

Band gap of the Si QD depends on size of the dots and quantum confinement parameter

Effective band gap


Wide band gap material

Top cell Middle cell c-Si cell

Si-QDs for PV applications

Si-QD

2 eV 1.5 eV 1.1 eV

Multi-junction solar cell of Si with control over Si-QD size is possible

Theoretical efficiency of triple junction cell is about 64%

dEh

mTe

∆−=∗

2

8exp16

Tunneling Probability

2)()(

d

kbulkEeVE gg +=

Band gap variation

Si QDs using porous SiTop down approach

Two approaches for obtaining Si QDs:

- Porous Si

- Thin film deposition (HWCVD)

Chetan S Solanki, IIT Bombay

Porosity, HF conc. & Current Density

HF solution

Silicon

H2

x = 0

x

ft(t)

Porosity (p) as a function of HF conc. and current density

Provides control over size of nanoparticles

Larger porosity results in smaller Si particle size

HFHF

HF

HF

15

35

55

75

0 50 100 150

Current Density (mA/cm2)

Po

rosi

ty (

%)

10%

12.5%

35%

25%

16.6%

HF


Porous Silicon (PS) Thin Films

Porous Silicon films between 5 to 15 micrometers are obtained

Time (min or sec)

Current Density(mA / cm2)

t1 t2

J1

J2

Pores

Pore diameter can be from nano-meter to micro-meter range

Use of heavily doped P-type Si results in nano-porous Si


2 Hr Sonication.

Particle size in range of

10nm.

TEM results

100 nm

Plan is:

Dispersion of Si particle in suitable dielectric

Spinning on substrate to deposit Si-QD layer

Control over particle size

Distribution of particles



Si Quantum dots using HWCVD

Bottom up approach

(Structural and optical characterization)

Hot Wire CVD

Wafer size 2 inch

Substrate temperature Room temperature to 800oC

Filament temperature Up to 2000oC

Gases SiH4, NH3, H2, B2H6, N2

Chamber pressure Up to 10-7 mbar

Distance between filament and substrate

3-5 cm

• SiH4 & NH3 cracked at hot filament

• Gas utilization:

~10% in PECVD

~80% in HWCVD

• Deposition rate as high as 2 nm/sec

5 cm

substrate

hot filamentsilane

ammonia

If

Principle of HWCVD


Variation in HWCVD parameters

Depo. parameters SiNx deposition Si-nS deposition

Basic Pressure ~ 10-6 mbar ~ 10-6 mbar

Gas pressure ~ 10-2 mbar ~ 10-3 mbar

Filament Temp. 1900oC 1900oC

Substrate Temp. 250oC 200-400-600-650-700-750oC

Gas Flow 1:20 sccm (SiH4:NH3) 1-2-4 sccm (SiH4)

Dep. Time 75 sec 60-40-20 sec

Deposition of Si-nS with variation in parameters to :

Substrate temperature

SiH4 & NH3 gas flow rate

Deposition time

to see the effects in Si-nS size

• With increasing above parameters, the Si-nS size expected to increase

Superlattice of SiNx/a-Si Using the optimized conditions 40 alternate layers of SiNx and a-Si are deposited

1 deposition takes about 5 hr

A novel deposition technique is developed where the flow of NH3 is interrupted for pre-determined time

Substrate

SiNxa-Si

SiNxa-Si

SiNxa-Si

40 layers

Substrate

SiNx

SiNx

SiNx40 layers

Annealinga-Si

a-Si

a-Si


Effect of annealing Temp.: 1st order Raman spectra

Annealing temperature of sample

(oC)

a-Si peak position

(cm-1)

Si-QD peak position

(cm-1)

Intensity ratio for 2nd order

Raman spectra

As-deposited 482 505 -

800 495 507 0.52

850 498 509 0.61

900 500 508 0.83

950 disappeared making shoulder

0.88

With increasing annealing temp.- a-Si phase reduces - Asymmetric shoulder appears

2nd order Raman spectra

Regain c-Si shape & increased intensity

Asymmetric shift & reduced intensity

With increasing annealing temp.

the 2nd order Raman peak for Si increases, shows the growth of Si-QD


PL Analysis

• Room temp. PL performed with He-Cd laser 325 nm

• PL peak shifts from blue to red wavelength as the Si-QD size increases as annealing temp. increases

• Estimated band gap between 2.3 to 2.6 eV

PL

inte

nsity

(a

.u.)

• With PL presence of quantum dots can be established


TEM analysis

a-Si/SiNx multilayer as-deposited

at 250oC and annealed at 850oC

for 30 min.

40 alternate layers are deposited

Thickness of layers in 4 to 6 nm

range

Optimized deposition and

annealing conditions were used

SIMS analysis is done for Si/N

variation in layers


TEM: as deposited multilayer

as-deposited a-Si/SiNx ML at 250oC

No Si-QD formation


Annealing of ML @850oC

annealed a-Si/SiNx multilayer at 850oC

Si-QD with av. Inter-dot distance < 5 nm

Optimized temp. for Si-QD growth

No Si precipitation in SiNx layer after annealing


Annealing of ML @900oC

annealed a-Si/SiNx multilayer at 900oC

Inter-dot distance < 1 nmmaking continuous nC Si film


38

Effect of deposition time of a-Si layer

A60AS

A20AS

A40AS

PL HRTEM

A20AS

A40AS

A60AS

a-Si ~3.0 nm

a-Si ~3.47 nm

No ML

2.44 eV

2.67 eV

At smaller deposition time, there is no continuity in a-Si layers

39

Solar cell with Si-QD/SiNx ML

QD

SiNx layer

n-Si substrate

Back Al

Front Al

i-type Si-QD/SiNx

p-type a-Si layer (30 nm)

Front Al

QD

QD

QD

QD

Solar cell device structure with Si quantum dots

Whole device fabricated in HWCVD

40

Cell with A60AS and A60AN850 40 ML

Voc

(mV)

Isc

(pA)

80 6.37

70 34.3

50 50

340 804

90 1.17

20 1.23

Hydrogenation for 40 min at 420oC after Al contact evaporation40 Si-QD/SiNx ML

as-deposited on

different devices

illuminated

40 Si-QD/SiNx ML

annealed gives no

I-V characteristics

with illumination

Best cell

Conclusion and Future Work Successful formation of less than 10 nm Si dots using PS

Successful development of alternate 40 layers of a-Si/SiNx in single HWCVD

Formation of Si-QD 3-5 nm with optimized annealing temp. 850oC

No Si-QD formed in as-deposited samples

Charge transport increased due to light shining on the device

Best cell with 340 mV of open circuit voltage

Effect of band gap enhancement on open circuit voltage is yet to be demonstrated

Current transport through quantum dot structure is under investigation


Thank you for your

attention

“It isn't new energy that will make such a difference in the next millennium. The power to run ……………... It's new technology that will bring proven ways of generating power ………………..”

IRCC Award Talk Si Quantum Dots for Solar Cell … of Energy Science and Engineering Indian...

Documents

Transcript of IRCC Award Talk Si Quantum Dots for Solar Cell … of Energy Science and Engineering Indian...